3GPP LTE employs Orthogonal Frequency Division Multiple Access (OFDMA) as a downlink communication scheme. In radio communication systems to which 3GPP LTE is applied, base stations transmit synchronization signals (i.e., Synchronization Channel: SCH) and broadcast signals (i.e., Broadcast Channel: BCH) using predetermined communication resources. Meanwhile, each terminal finds an SCH first and thereby ensures synchronization with the base station. Subsequently, the terminal reads BCH information to acquire base station-specific parameters (e.g., frequency bandwidth) (see, Non-Patent Literatures (hereinafter, abbreviated as NPL) 1, 2 and 3).
In addition, upon completion of the acquisition of the base station-specific parameters, each terminal sends a connection request to the base station to thereby establish a communication link with the base station. The base station transmits control information via Physical Downlink Control CHannel (PDCCH) as appropriate to the terminal with which a communication link has been established via a downlink control channel or the like.
The terminal performs “blind-determination” on each of a plurality of pieces of control information included in the received PDCCH signal (i.e., Downlink (DL) Assignment Control Information: also referred to as Downlink Control Information (DCI)). To put it more specifically, each piece of the control information includes a Cyclic Redundancy Check (CRC) part and the base station masks this CRC part using the terminal ID of the transmission target terminal. Accordingly, until the terminal demasks the CRC part of the received piece of control information with its own terminal ID, the terminal cannot determine whether or not the piece of control information is intended for the terminal. In this blind-determination, if the result of demasking the CRC part reports that the CRC operation is OK, the piece of control information is determined as being intended for the terminal.
Moreover, in 3GPP LTE, Automatic Repeat Request (ARQ) is applied to downlink data to terminals from a base station. To put it more specifically, each terminal feeds back a response signal indicating the result of error detection on the downlink data to the base station. Each terminal performs a CRC on the downlink data and feeds back Acknowledgment (ACK) when CRC=OK (no error) or Negative Acknowledgment (NACK) when CRC=Not OK (error) to the base station as a response signal. An uplink control channel such as Physical Uplink Control Channel (PUCCH) is used to feed back the response signals (i.e., ACK/NACK signals (hereinafter, may be referred to as “A/N,” simply)).
The control information to be transmitted from a base station herein includes resource assignment information including information on resources assigned to the terminal by the base station. As described above, PDCCH is used to transmit this control information. This PDCCH includes one or more L1/L2 control channels (L1/L2 CCH). Each L1/L2 CCH consists of one or more Control Channel Elements (CCE). To put it more specifically, a CCE is the basic unit used to map the control information to PDCCH. Moreover, when a single L1/L2 CCH consists of a plurality of CCEs (2, 4 or 8), a plurality of contiguous CCEs starting from a CCE having an even index are assigned to the L1/L2 CCH. The base station assigns the L1/L2 CCH to the resource assignment target terminal in accordance with the number of CCEs required for indicating the control information to the resource assignment target terminal. The base station maps the control information to physical resources corresponding to the CCEs of the L1/L2 CCH and transmits the mapped control information.
In addition, CCEs are associated with component resources of PUCCH (hereinafter, may be referred to as “PUCCH resource”) in a one-to-one correspondence. Accordingly, a terminal that has received an L1/L2 CCH identifies the component resources of PUCCH that correspond to the CCEs forming the L1/L2 CCH and transmits a response signal to the base station using the identified resources. However, when the L1/L2 CCH occupies a plurality of contiguous CCEs, the terminal transmits the response signal to the base station using a PUCCH component resource corresponding to a CCE having a smallest index among the plurality of PUCCH component resources respectively corresponding to the plurality of CCEs (i.e., PUCCH component resource associated with a CCE having an even numbered CCE index). In this manner, the downlink communication resources are efficiently used.
As illustrated in FIG. 1, a plurality of response signals transmitted from a plurality of terminals are spread using a Zero Auto-correlation (ZAC) sequence having the characteristic of zero autocorrelation in time-domain, a Walsh sequence and a discrete Fourier transform (DFT) sequence, and are code-multiplexed in a PUCCH. In FIG. 1, (W0, W1, W2, W3) represent a length-4 Walsh sequence and (F0, F1, F2) represent a length-3 DFT sequence. As illustrated in FIG. 1, ACK or NACK response signals are primary-spread over frequency components corresponding to 1 SC-FDMA symbol by a ZAC sequence (length-12) in frequency-domain. To put it more specifically, the length-12 ZAC sequence is multiplied by a response signal component represented by a complex number. Subsequently, the ZAC sequence serving as the response signals and reference signals after the primary-spread is secondary-spread in association with each of a Walsh sequence (length-4: W0-W3 (may be referred to as Walsh Code Sequence)) and a DFT sequence (length-3: F0-F2). To put it more specifically, each component of the signals of length-12 (i.e., response signals after primary-spread or ZAC sequence serving as reference signals (i.e., Reference Signal Sequence) is multiplied by each component of an orthogonal code sequence (i.e., orthogonal sequence: Walsh sequence or DFT sequence). Moreover, the secondary-spread signals are transformed into signals of length-12 in the time-domain by inverse fast Fourier transform (IFFT). A CP is added to each signal obtained by IFFT processing, and the signals of one slot consisting of seven SC-FDMA symbols are thus formed.
The response signals from different terminals are spread using ZAC sequences each corresponding to a different cyclic shift value (i.e., index) or orthogonal code sequences each corresponding to a different sequence number (i.e., orthogonal cover index (OC index)). An orthogonal code sequence is a combination of a Walsh sequence and a DFT sequence. In addition, an orthogonal code sequence is referred to as a block-wise spreading code in some cases. Thus, base stations can demultiplex the code-multiplexed plurality of response signals using the related art despreading and correlation processing (see, NPL 4).
However, it is not necessarily true that each terminal succeeds in receiving downlink assignment control signals because the terminal performs blind-determination in each subframe to find downlink assignment control signals intended for the terminal. When the terminal fails to receive the downlink assignment control signals intended for the terminal on a certain downlink component carrier, the terminal would not even know whether or not there is downlink data intended for the terminal on the downlink component carrier. Accordingly, when a terminal fails to receive the downlink assignment control signals intended for the terminal on a certain downlink component carrier, the terminal generates no response signals for the downlink data on the downlink component carrier. This error case is defined as discontinuous transmission of ACK/NACK signals (DTX of response signals) in the sense that the terminal transmits no response signals.
In 3GPP LTE systems (may be referred to as “LTE system,” hereinafter), base stations assign resources to uplink data and downlink data, independently. For this reason, in the 3GPP LTE system, terminals (i.e., terminals compliant with LTE system (hereinafter, referred to as “LTE terminal”)) encounter a situation where the terminals need to transmit uplink data and response signals for downlink data simultaneously in the uplink. In this situation, the response signals and uplink data from the terminals are transmitted using time-division multiplexing (TDM). As described above, the single carrier properties of transmission waveforms of the terminals are maintained by the simultaneous transmission of response signals and uplink data using TDM.
In addition, as illustrated in FIG. 2, the response signals (i.e., “A/N”) transmitted from each terminal partially occupy the resources assigned to uplink data (i.e., Physical Uplink Shared CHannel (PUSCH) resources) (i.e., response signals occupy some SC-FDMA symbols adjacent to SC-FDMA symbols to which reference signals (RS) are mapped) and are thereby transmitted to a base station in time-division multiplexing (TDM). However, “subcarriers” in the vertical axis in FIG. 2 are also termed as “virtual subcarriers” or “time contiguous signals,” and “time contiguous signals” that are collectively inputted to a discrete Fourier transform (DFT) circuit in a SC-FDMA transmitter are represented as “subcarriers” for convenience. To put it more specifically, optional data of the uplink data is punctured due to the response signals in the PUSCH resources. Accordingly, the quality of uplink data (e.g., coding gain) is significantly reduced due to the punctured bits of the coded uplink data. For this reason, base stations instruct the terminals to use a very low coding rate and/or to use very large transmission power so as to compensate for the reduced quality of the uplink data due to the puncturing.
Meanwhile, the standardization of 3GPP LTE-Advanced for realizing faster communication than 3GPP LTE is in progress. 3GPP LTE-Advanced systems (may be referred to as “LTE-A system,” hereinafter) follow LTE systems. 3GPP LTE-Advanced will introduce base stations and terminals capable of communicating with each other using a wideband frequency of 40 MHz or greater to realize a downlink transmission rate of up to 1 Gbps or above.
In the LTE-A system, in order to simultaneously achieve backward compatibility with the LTE system and ultra-high-speed communication several times faster than transmission rates in the LTE system, the LTE-A system band is divided into “component carriers” of 20 MHz or below, which is the bandwidth supported by the LTE system. In other words, the “component carrier” is defined herein as a band having a maximum width of 20 MHz and as the basic unit of communication band. In the Frequency Division Duplex (FDD) system, moreover, “component carrier” in downlink (hereinafter, referred to as “downlink component carrier”) is defined as a band obtained by dividing a band according to downlink frequency bandwidth information in a BCH broadcasted from a base station or as a band defined by a distribution width when a downlink control channel (PDCCH) is distributed in the frequency domain. In addition, “component carrier” in uplink (hereinafter, referred to as “uplink component carrier”) may be defined as a band obtained by dividing a band according to uplink frequency band information in a BCH broadcasted from a base station or as the basic unit of a communication band of 20 MHz or below including a Physical Uplink Shared CHannel (PUSCH) in the vicinity of the center of the bandwidth and PUCCHs for LTE on both ends of the band. In addition, the term “component carrier” may be also referred to as “cell” in English in 3GPP LTE-Advanced. Furthermore, “component carrier” may also be abbreviated as CC(s).
In the Time Division Duplex (TDD) system, a downlink component carrier and an uplink component carrier have the same frequency band, and downlink communication and uplink communication are realized by switching between the downlink and uplink on a time division basis. For this reason, in the case of the TDD system, the downlink component carrier can also be expressed as “downlink communication timing in a component carrier.” The uplink component carrier can also be expressed as “uplink communication timing in a component carrier.” The downlink component carrier and the uplink component carrier are switched based on a UL-DL configuration as shown in FIG. 3. In the UL-DL configuration shown in FIG. 3, timings are configured in subframe units (that is, 1 msec units) for downlink communication (DL) and uplink communication (UL) per frame (10 msec). The UL-DL configuration can construct a communication system capable of flexibly meeting a downlink communication throughput requirement and an uplink communication throughput requirement by changing a subframe ratio between downlink communication and uplink communication. For example, FIG. 3 illustrates UL-DL configurations (Config 0 to 6) having different subframe ratios between downlink communication and uplink communication. In addition, in FIG. 3, a downlink communication subframe is represented by “D,” an uplink communication subframe is represented by “U” and a special subframe is represented by “S.” Here, the special subframe is a subframe at the time of switchover from a downlink communication subframe to an uplink communication subframe. In the special subframe, downlink data communication may be performed as in the case of the downlink communication subframe. In each UL-DL configuration shown in FIG. 3, subframes (20 subframes) corresponding to 2 frames are expressed in two stages: subframes (“D” and “S” in the upper row) used for downlink communication and subframes (“U” in the lower row) used for uplink communication. Furthermore, as shown in FIG. 3, an error detection result corresponding to downlink data (ACK/NACK) is reported in the fourth uplink communication subframe or an uplink communication subframe after the fourth subframe after the subframe to which the downlink data is assigned.
The LTE-A system supports communication using a band obtained by bundling some component carriers, so-called carrier aggregation (CA). Note that while a UL-DL configuration can be set for each component carrier, an LTE-A system compliant terminal (hereinafter, referred to as “LTE-A terminal”) is designed assuming that the same UL-DL configuration is set among a plurality of component carriers.
FIGS. 4A and 4B are diagrams provided for describing asymmetric carrier aggregation and a control sequence thereof applicable to individual terminals.
As illustrated in FIG. 4B, a configuration in which carrier aggregation is performed using two downlink component carriers and one uplink component carrier on the left is set for terminal 1, while a configuration in which the two downlink component carriers identical with those used by terminal 1 are used but uplink component carrier on the right is used for uplink communication is set for terminal 2.
Referring to terminal 1, a base station included an LTE-A system (that is, LTE-A system compliant base station (hereinafter, referred to as “LTE-A base station”) and an LTE-A terminal included in the LTE-A system transmit and receive signals to and from each other in accordance with the sequence diagram illustrated in FIG. 4A. As illustrated in FIG. 4A, (1) terminal 1 is synchronized with the downlink component carrier on the left when starting communications with the base station and reads information on the uplink component carrier paired with the downlink component carrier on the left from a broadcast signal called system information block type 2 (SIB2). (2) Using this uplink component carrier, terminal 1 starts communication with the base station by transmitting, for example, a connection request to the base station. (3) Upon determining that a plurality of downlink component carriers need to be assigned to the terminal, the base station instructs the terminal to add a downlink component carrier. However, in this case, the number of uplink component carriers does not increase, and terminal 1, which is an individual terminal, starts asymmetric carrier aggregation.
In addition, in the LTE-A system to which carrier aggregation is applied, a terminal may receive a plurality of pieces of downlink data on a plurality of downlink component carriers at a time. In LTE-A, channel selection (also referred to as “multiplexing”), bundling and a discrete Fourier transform spread orthogonal frequency division multiplexing (DFT-S-OFDM) format are available as a method of transmitting a plurality of response signals for the plurality of pieces of downlink data. In channel selection, a terminal causes not only symbol points used for response signals, but also the resources to which the response signals are mapped to vary in accordance with the pattern for results of the error detection on the plurality of pieces of downlink data. Compared with channel selection, in bundling, the terminal bundles ACK or NACK signals generated according to the results of error detection on the plurality of pieces of downlink data (i.e., by calculating a logical AND of the results of error detection on the plurality of pieces of downlink data, provided that ACK=1 and NACK=0), and response signals are transmitted using one predetermine resource. In transmission using the DFT-S-OFDM format, a terminal jointly encodes (i.e., joint coding) the response signals for the plurality of pieces of downlink data and transmits the coded data using the format (see, NPL 5). For example, a terminal may feed back the response signals (i.e., ACK/NACK) using channel selection, bundling or DFT-S-OFDM according to the number of bits for a pattern for results of error detection. Alternatively, a base station may previously configure the method of transmitting the response signals.
Channel Selection is a technique that varies not only the phase points (i.e., constellation points) for the response signals but also the resources used for transmission of the response signals (may be referred to as “PUCCH resource,” hereinafter) on the basis of whether the results of error detection on the plurality of pieces of downlink data for each downlink component carrier received on the plurality of downlink component carriers (a maximum of two downlink component carriers) are each an ACK or NACK as illustrated in FIG. 5. Meanwhile, bundling is a technique that bundles ACK/NACK signals for the plurality of pieces of downlink data into a single set of signals and thereby transmits the bundled signals using one predetermined resource (see, NPLs 6 and 7). Hereinafter, the set of the signals formed by bundling ACK/NACK signals for a plurality of pieces of downlink data into a single set of signals may be referred to as “bundled ACK/NACK signals.”
The following two methods are considered as a possible method of transmitting response signals in uplink when a terminal receives downlink assignment control information via a PDCCH and receives downlink data.
One of the methods is to transmit response signals using a PUCCH resource associated in a one-to-one correspondence with a control channel element (CCE) occupied by the PDCCH (i.e., implicit signaling) (hereinafter, method 1). More specifically, when DCI intended for a terminal served by a base station is mapped in a PDCCH region, each PDCCH occupies a resource consisting of one or a plurality of contiguous CCEs. In addition, as the number of CCEs occupied by a PDCCH (i.e., the number of aggregated CCEs: CCE aggregation level), one of aggregation levels 1, 2, 4 and 8 is selected according to the number of information bits of the assignment control information or a propagation path condition of the terminal, for example.
The other method is to previously indicate a PUCCH resource to each terminal from a base station (i.e., explicit signaling) (hereinafter, method 2). To put it differently, each terminal transmits response signals using the PUCCH resource previously indicated by the base station in method 2.
Furthermore, as shown in FIG. 5, the terminal transmits response signals using one of two component carriers. A component carrier that transmits such response signals is called “primary component carrier (PCC) or primary cell (PCell).” The other component carrier is called “secondary component carrier (SCC) or secondary cell (SCell).” For example, the PCC (PCell) is a component carrier that transmits broadcast information on a component carrier that transmits response signals (e.g., system information block type 2 (SIB2)).
In method 2, PUCCH resources common to a plurality of terminals (e.g., four PUCCH resources) may be previously indicated to the terminals from a base station. For example, terminals may employ a method to select one PUCCH resource to be actually used, on the basis of a transmit power control (TPC) command of two bits included in DCI in SCell. In this case, the TPC command is also called an ACK/NACK resource indicator (ARI). Such a TPC command allows a certain terminal to use an explicitly signaled PUCCH resource in a certain subframe while allowing another terminal to use the same explicitly signaled PUCCH resource in another subframe in the case of explicit signaling.
Meanwhile, in channel selection, a PUCCH resource in an uplink component carrier associated in a one-to-one correspondence with the top CCE index of the CCEs occupied by the PDCCH indicating the PDSCH in PCC (PCell) (i.e., PUCCH resource in PUCCH region 1 in FIG. 5) is assigned (implicit signaling).
Here, ARQ control using channel selection when the above asymmetric carrier aggregation is applied to a terminal will be described with reference to FIG. 5 and FIGS. 6A and 6B.
For example, in FIG. 5, a component carrier group (may be referred to as “component carrier set” in English) consisting of component carrier 1 (PCell) and component carrier 2 (SCell) is set for terminal 1. In this case, after downlink resource assignment information is transmitted to terminal 1 from the base station via a PDCCH of each of component carriers 1 and 2, downlink data is transmitted using the resource corresponding to the downlink resource assignment information.
Furthermore, in channel selection, response signals representing error detection results corresponding to a plurality of pieces of downlink data in component carrier 1 (PCell) and error detection results corresponding to a plurality of pieces of downlink data in component carrier 2 (SCell) are mapped to PUCCH resources included in PUCCH region 1 or PUCCH region 2. The terminal uses two types of phase points (Binary Phase Shift Keying (BPSK) mapping) or four types of phase points (Quadrature Phase Shift Keying (QPSK) mapping) as response signals thereof. That is, in channel selection, it is possible to express a pattern for results of error detection corresponding to a plurality of pieces of downlink data in component carrier 1 (PCell) and the results of error detection corresponding to a plurality of pieces of downlink data in component carrier 2 (SCell) by a combination of PUCCH resources and phase points.
Here, FIG. 6A shows a method of mapping a pattern for results of error detection when the number of component carriers is two (one PCell, one SCell) in a TDD system.
Note that FIG. 6A assumes a case where the transmission mode is set to one of (a), (b) and (c) below.                (a) A transmission mode in which each component carrier supports only one-CW transmission in downlink        (b) A transmission mode in which one component carrier supports only one-CW transmission in downlink and the other component carrier supports up to two-CW transmission in downlink        (c) A transmission mode in which each component carrier supports up to two-CW transmission in downlink        
Furthermore, FIG. 6A assumes a case where number M is set in one of (1) to (4) below, M indicating how many downlink communication subframes per component carrier (hereinafter, described as “DL (DownLink) subframes,” “D” or “S” shown in FIG. 3) of results of error detection need to be reported to the base station using one uplink communication subframe (hereinafter, described as “UL (UpLink) subframe,” “U” shown in FIG. 3). For example, in Config 2 shown in FIG. 3, since results of error detection of four DL subframes are reported to the base station using one UL subframe, M=4.                (1) M=1        (2) M=2        (3) M=3        (4) M=4        
That is, FIG. 6A illustrates a method of mapping a pattern for results of error detection when (a) to (c) above are combined with (1) to (4) above. The value of M varies depending on UL-DL configuration (Config 0 to 6) and subframe number (SF#0 to SF#9) in one frame as shown in FIG. 3. Furthermore, in Config 5 shown in FIG. 3, M=9 in subframe (SF) #2. However, in this case, in the LTE-A TDD system, the terminal does not apply channel selection and reports the results of error detection using, for example, a DFT-S-OFDM format. For this reason, in FIG. 6A, Config 5 (M=9) is not included in the combination.
In the case of (1), the number of error detection result patterns is 22×1=4 patterns, 23×1=8 patterns and 24×1=16 patterns in order of (a), (b) and (c). In the case of (2), the number of error detection result patterns is 22×2=8 patterns, 23×2=16 patterns, 24×2=32 patterns in order of (a), (b) and (c). The same applies to (3) and (4).
Here, it is assumed that the phase difference between phase points to be mapped in one PUCCH resource is 90 degrees at minimum (that is, a case where a maximum of 4 patterns per PUCCH resource are mapped). In this case, the number of PUCCH resources necessary to map all error detection result patterns is 24×4÷4=16 in (4) and (c) when the number of error detection result patterns is a maximum (24×4=64 patterns), which is not realistic. Thus, the TDD system intentionally reduces the amount of information on the results of error detection by bundling the results of error detection in a spatial region or further in a time domain if necessary. In this way, the TDD system limits the number of PUCCH resources necessary to report the error detection result patterns.
In the LTE-A TDD system, in the case of (1), the terminal maps 4 patterns, 8 patterns and 16 patterns of results of error detection in order of (a), (b) and (c) to 2, 3 and 4 PUCCH resources respectively without bundling the results of error detection (Step3 in FIG. 6A). That is, the terminal reports an error detection result using 1 bit per component carrier in which a transmission mode (non-MIMO) supporting only one-codeword (CW) transmission in downlink and reports error detection results using 2 bits per component carrier in which a transmission mode (MIMO) supporting up to two-CW transmissions in downlink.
In the LTE-A TDD system, in the cases of (2) and (a), the terminal maps eight patterns of results of error detection to four PUCCH resources without bundling the results of error detection (Step3 in FIG. 6A). In that case, the terminal reports error detection results using 2 bits per downlink component carrier.
In the LTE-A TDD system, in the cases of (2) and (b) (the same applies to (2) and (c)), the terminal bundles the results of error detection of component carriers in which a transmission mode supporting up to two-CW transmission in downlink is set in a spatial region (spatial bundling) (Step1 in FIG. 6A). In the spatial bundling, when the result of error detection corresponding to at least one CW of two CWs of the results of error detection is NACK, the terminal determines the results of error detection after the spatial bundling to be NACK. That is, in spatial bundling, Logical And of the results of error detection of two CWs is taken. The terminal then maps error detection result patterns after spatial bundling (8 patterns in the cases of (2) and (b), 16 patterns in the cases of (2) and (c)) to four PUCCH resources (Step3 in FIG. 6A). In that case, the terminal reports error detection results using 2 bits per downlink component carrier.
In the LTE-A TDD system, in the cases of (3) or (4), and (a), (b) or (c), the terminal performs bundling in the time domain (time-domain bundling) after the spatial bundling (Step1) (Step2 in FIG. 6A). The terminal then maps the error detection result patterns after the time-domain bundling to four PUCCH resources (Step3 in FIG. 6A). In that case, the terminal reports results of error detection using 2 bits per downlink component carrier.
Next, an example of more specific mapping methods will be described with reference to FIG. 6B. FIG. 6B shows an example of a case where the number of downlink component carriers is 2 (one PCell, one SCell) and a case where “(c) a transmission mode in which each component carrier supports up to two-CW transmission in the downlink” is set and a case with “(4) M=4.”
In FIG. 6B, the results of error detection of a PCell are (ACK (A), ACK), (ACK, ACK), (NACK (N), NACK) and (ACK, ACK) in order of (CW0, CW1) in four DL subframes (SF1 to 4). In the PCell shown in FIG. 6B, M=4, and therefore the terminal spatially bundles these subframes in Step1 in FIG. 6A (portions enclosed by a solid line in FIG. 6B). As a result of the spatial bundling, ACK, ACK, NACK and ACK are obtained in that order in four DL subframes of the PCell shown in FIG. 6B. Furthermore, in Step2 in FIG. 6A, the terminal applies time-domain bundling to the 4-bit error detection result pattern (ACK, ACK, NACK, ACK) after spatial bundling obtained in Step1 (portions enclosed by broken line in FIG. 6B). In this way, a 2-bit error detection result of (NACK, ACK) is obtained in the PCell shown in FIG. 6B.
The terminal likewise applies spatial bundling and time-domain bundling also for the SCell shown in FIG. 6B and thereby obtains a 2-bit error detection result (NACK, NACK).
The terminal then combines the error detection result patterns using 2 bits each after time-domain bundling of the PCell and SCell in Step3 in FIG. 6A in order of the PCell, SCell to bundle them into a 4-bit error detection result pattern (NACK, ACK, NACK, NACK). The terminal determines a PUCCH resource (in this case, h1) and a phase point (in this case, −j) using the mapping table shown in Step3 in FIG. 6A from this 4-bit error detection result pattern.